Hypolimnetic withdrawal in two north american lakes with anoxic phosphorus release from the sediment

Abstract Hypolimnetic withdrawal has been used to decrease eutrophication in two Connecticut lakes. This restoration technique is based on the forced discharge of nutrient-rich bottom waters in lakes with anoxic hypolimnia, while surface outflows are dammed. Internal phosphorus load amounted to 600 mg m −2 summer −1 before withdrawal. Most of this load probably originated from the sediment since the experimentally determined sediment phosphorus release rates of 2–12 mg m −2 day −1 can account for the internal load. The smaller lake responded to hypolimnetic withdrawal with decreasing epilimnetic and hypolimnetic phosphorus concentrations, decreased anoxia and internal loads. The larger lake also showed a tendency towards improvement which is, however, not statistically significant.